1. Technical Field
The present invention relates to touchpad pointing devices, and in particular to a touchpad pointing device which is less expensive to fabricate.
2. Summary of the Prior Art
Over the last several years, capacitive touchpad pointing devices have been used extensively in personal computers. When used in conjunction with a computer, a touchpad allows the user to manipulate a graphics cursor on a CRT display. The touchpad comprises a sensitive planar surface and means for detecting the position of an object, such as a finger or a stylus, near or in contact with, the sensitive planar surface. The touchpad continuously communicates this position information to the electronic apparatus, typically at a rate of from 40 to 100 Hz.
Presently, approximately 70% of all new notebook computers include a touchpad as the primary pointing device. In this competitive touchpad market, price has become an increasingly important factor. A touchpad design that is intrinsically less expensive to make has significant commercial advantage.
For reasons to be disclosed below, most existing capacitive touchpad designs are built using a four-layer printed circuit board as the basic structural substrate. The circuit traces on the "top" two (finger-side) layers of the board serve as the capacitive sensing matrix, and the traces on the "bottom", or underside (component-side) layer provide electrical interconnections for the components mounted thereon. The remaining buried layer, the fourth layer, is generally used as a ground plane though it could, in theory, be put to some other use.
While four-layer printed circuit boards are widely available from many manufacturers, it is well-known that they are substantially more expensive than two-layer printed circuit boards of the same size. In large quantities, in excess of 200,000 units per month, a four-layer touchpad-sized printed circuit board with dimensions of, for example, 65 mm by 49 mm might typically cost $0.80 per unit, with the exact figure depending on many factors. A two-layer circuit board of the same size might cost as little as approximately $0.40. This savings is substantial when compared to the overall cost of the entire touchpad device. The ability to use a two-layer instead of four-layer circuit board would tend to give a strong competitive advantage to a manufacturer capable of producing such a device.
Two-layer touchpads have been constructed for limited applications. As will be described in greater detail below, these two-layer touchpads require that the controller chip be remotely located on the same circuit board away from the touch sensitive area. Thus, such two-layer touchpads do not perform an equivalent function as do present four-layer touchpads.